1. Field of the Invention
The present invention relates primarily to the field of electrosurgery. In particular, the present invention relates to the field of radiofrequency (RF) electrosurgery, including RF cautery, RF dissection, RF fulguration or coagulation, RF desiccation or any other forms of RF-assisted electrosurgery.
2. Description of the Related Art
Electrosurgery generally refers to the use of electrically energized surgical devices to operate upon a patient. Common electrosurgical devices include electrocautery devices in which a direct current (DC) is caused to flow through a wire loop. The resistance of the wire causes the loop to heat up, thereby enabling the cauterization of the tissue.
More commonly, electrosurgical devices (whether monopolar or bipolar) utilize alternating current (AC) in the range of about 200 kHz to about 3.3 MHz (hereafter “RF”) that is applied to the patient via a RF electrode. Such RF electrosurgical instruments are typically used to dissect (cut), fulgurate (coagulate) or desiccate (dry out) tissue by selectively varying the power and duty cycle of the RF signal applied to the electrode of the electrosurgical instrument. When the RF electrosurgical instrument is monopolar, a large low impedance return (dispersive) electrode is affixed to the patient to provide the current return path to ground. A bipolar RF electrosurgical instrument has two electrodes, the active electrode and the return electrode. The flow of current occurs between these two electrodes, thus obviating the need for a large low impedance return electrode. An insulated RF generator is typically used to energize the RF electrode(s) of the RF electrosurgical instrument.
The density of the current delivered to the active electrode may be affected by the impedance (a vector quantity consisting of the tissue's resistance and its reactance) of the tissue. The impedance of the tissue to which the electrodes are exposed has an important effect upon the functioning of RF electrosurgical devices. This is because the tissue becomes an integral part of the circuit comprising the RF generator, the active electrode, the patient's tissue, the return electrode and back to the RF generator to ground or a reference voltage. Therefore, a voltage controlled or limited RF generator (generally providing power between about 10 W and 200 W) is generally used, so that when the impedance of tissue rises, the current delivered to the active electrode safely decreases. The density and impedance of tissue affects the manner in which it is affected by the active electrode. Tissues with higher salt content and body fluids such as blood have a high conductance and therefore, lower impedance. Fatty tissues have a lower fluid content and generally exhibit relatively higher impedance than leaner tissues. Thus, for a given voltage, fatty tissue will dissipate less current than leaner tissue. Highly vascularized tissues, on the other hand, have very low impedance and tend to conduct current more efficiently than less vascularized tissues. Therefore, to achieve the same tissue effect, current must be applied to a RF electrode in contact with highly vascularized tissue for a longer period of time than would be the case if the RF electrode were in contact with comparatively less vascularized tissue. Alternatively, the initial power applied to the RF electrode may need to be momentarily increased to achieve the same tissue effect in a fixed period of time.
FIGS. 1–3 show a RF excisional device 100 that includes a probe 102, the distal end 108 of which includes a conductive ribbon or wire loop 106 configured to bow out of and back into a window 104 defined within the probe 102. The loop 106 is connected to a RF power supply (not shown). The loop 106 is energized with RF energy and the probe 102 is rotated as shown at 112 with the loop 106 in an extended or deployed configuration, as shown in FIG. 3. As the probe 102 is rotated, a volume of revolution of tissue is cut by the RF-energized loop 106.
However, when the loop 106 is in contact with the tissue to be cut, the physician often must wait a relatively long period of time (several seconds or longer) between initial application of the RF power to the loop 106 and the time at which the current density within the loop 106 is high enough to cut (vaporize) the tissue. This is because the relatively low impedance of the interface between the loop 106 and the patient's tissue shown at 114 in FIG. 5 dissipates a substantial amount of the current delivered to the loop 106. Highly vascularized tissue may also increase the current conduction (symbolized by the arrows emanating from the loop 106 in FIG. 5) away from the loop 106 and further delay the time at which the RF loop 106 is able to cut through the tissue 114. Indeed, until the tissue 114 chars and the impedance thereof increases, the current density within the loop 106 may not quickly reach that level at which the energized loop 106 is therapeutically effective.
One solution to this problem is to initially increase or surge the power delivered to the loop 106 to decrease the time required for the current density therein to reach the desired level. However, this is a less than optimal solution, as such an initial power surge may cause pain, excessive charring of the tissue 114 at the initial cut site, and may present a safety risk to the patient, who is better served by maintaining power levels as low as practicable to achieve the objective of the procedure. Such initial power surges, moreover, require specialized RF generators that are configured to deliver such surges safely and on demand.
What are needed, therefore, are improved methods and RF electrosurgical devices that do not suffer from the aforementioned disadvantages. More particularly, what are needed are RF electrosurgical devices that energize quickly, cut more efficiently and less painfully, reduce tissue charring at the initial cut site, and do not require an initial power surge from a specialized RF generator.